Coal chemical engineering is a process in which coal is used as the raw material and processed chemically into gas, liquid or solid fuel and chemical products. The coal chemical engineering mainly includes coal gasification, liquefaction, carbonization, and tar processing and carbide acetylene processing, etc. Coal chemical projects consume water heavily and produce a large quantity of waste water. However, in China, most coal chemical projects are located in the Northwest China region, where the water resources and pollutant receiving water bodies are deficient. As the environmental protection policies of the state are adjusted continuously, the waste water produced in coal chemical enterprises must be reused as far as possible to achieve zero release. The waste water in the coal chemical engineering usually can be categorized into organic waste water and brine waste water, wherein, the brine waste water mainly include circulating waste water and drainage water from chemical water stations, etc. To reuse the waste water as far as possible, usually the organic waste water is also introduced into the brine waste water treatment system for further demineralization treatment after primary treatment, secondary treatment, and advanced treatment, to ensure that the treated organic waste water meets the requirement for water quality of circulating water and replenishing water.
The brine waste water recycling treatment mainly includes two types of techniques: membrane separation techniques and thermal evaporation techniques. Owing to the fact that the approach of obtaining reused water directly from brine waste water with thermal evaporation techniques involves huge energy consumption, membrane separation techniques are the mainstream techniques in the art. The major membrane for separation techniques include osmotic membrane (reverse osmosis, forward osmosis), bipolar membrane, dialysis membrane, electrodialysis membrane, microwave membrane, ultrafiltration membrane, nanofiltration membrane, and biological membrane, etc. Usually one of the membrane techniques can be used separately, or several membrane techniques can be used in combination for the treatment, depending on the source of the waste water. A key technical challenge for membrane techniques is the membrane contamination problem, which has severe impact on the service efficiency and service life of the membranes owing to the complex composition of the waste water.
In addition, the objective of the brine waste water treatment techniques is zero release of waste water, which requires that the salts in the waste water should be recovered in the form of solid. Owing to the complex salt components in the waste water, it is difficult to obtain salts that meet the quality criteria for sale. The salts obtained with most of the existing methods are mixed salts, which are difficult to sell and use; consequently, solid wastes that are difficult to handle are formed. Though the salts in the waste water may be treated respectively with different separation and purification methods theoretically to obtain high-purity single salts, the process is complex and requires increased cost, and is not affordable in the economic aspect, owing to a large quantity of components. Especially, the existence of organic contaminants and nitrogenous contaminants in the waste water not only bring membrane contamination, but also introduce difficulties in the comprehensive treatment and recycling of mixed salts; as a result, true near-zero release can't be realized. Therefore, treating the organic contaminants and nitrogenous contaminants in the brine waste water efficiently with appropriate techniques is a prerequisite for steady operation of the follow-up membrane reduction units, and comprehensive utilization of the resultant mixed salts may be possible only if the pollutant concentration in the concentrated liquid is decreased as far as possible.
The patent document CN104016529A has disclosed a method for treatment of brine waste water based on a multi-stage reflux inverse-pole electrodialyzer in coal chemical engineering, which can concentrate the concentrated water by 10 times or more times and can improve the fresh water production efficiency to 85% or higher. By means of ozone oxidation, multi-stage membrane filtering and multi-stage reflux inverse-pole electrodialysis, the technique improves the fresh water recovery efficiency, and alleviates the problem of electrodialyzer membrane contamination to some extent, but the pretreatment cost is high, the stability of the pretreatment membranes is poor, the reused water obtained with the electrodialyzer still contains some salts, the overall effect is unsatisfactory, and there is no treatment solution for the strong brine, and zero release is not realized. The patent document CN104230124A has disclosed a process for sorted collection and separate treatment of waste water according to the quality of the waste water in coal chemical engineering, as well as a special device, which can be used to obtain industrial salts while improving the recovery efficiency of water. However, in that process, three reverse osmosis units are used separately, and advanced pretreatment measures such as ion exchange are required for pretreatment; at present, using a membrane separation unit to separate purified water and organic concentrated liquid is difficult to attain an ideal effect; the resultant industrial salts are mixed salts, which are difficult to sell or use.
The patent document CN103508602A has disclosed a process for zero-release treatment of high-salinity industrial waste water that integrates membrane treatment and evaporating crystallization. The industrial waste water is pretreated by ultrafiltration and then fed by a high-pressure pump to a reverse osmosis unit, the water outputted from the osmosis side is reused, and the concentrated liquid produced after several filtering cycles is treated by electrodialysis treatment, the material concentrated by electrodialysis is treated by evaporation and crystallization to obtain brine sludge and condensed water, the brine sludge is post-treated, the condensed water is reused, and the fresh water produced through electrodialysis is reused. That method uses reverse osmosis and electrodialysis in simple combination, so that reverse osmosis and electrodialysis play their own roles respectively. However, it is difficult to obtain fresh water that meets the criteria for reuse through electrodialysis, and the process involves high energy consumption and severe membrane contamination; the stability of the entire device is poor; in addition, the resultant solid salts are still mixed salts.
The patent document CN105565569A has disclosed enhanced advanced concentration system and process for high-salinity industrial waste water, in which high-salinity industrial waste water is conditioned in a conditioning tank, settled in a softening sedimentation tank, filtered in a V-type filtering tank, further filtered in an ultrafiltration device, concentrated in a primary reverse osmosis unit, deionized in a deionizer, and separated in a nanofiltration device, wherein: the concentrated water produced in the nanofiltration device is concentrated in a frequently changing pole electrodialysis device, the water produced in the frequently changing pole electrodialysis device is oxidized in an advanced oxidation device and then is sent to a main water tank, and the concentrated water is crystallized in a freezing crystallization system to obtain sodium sulfate crystals; the water produced in the nanofiltration device is concentrated in a secondary reverse osmosis unit, and then is further concentrated in a frequently changing pole electrodialysis device, the water produced in the frequently changing pole electrodialysis device is oxidized in an advanced oxidation device and then is sent to the main water tank, and the concentrated water is crystallized in a MVR evaporating crystallization device to obtain sodium chloride crystals. That method combines membrane techniques including nanofiltration, frequently changing pole electrodialysis, and reverse osmosis, etc., but hasn't effectively integrated the effects of different membrane techniques; the pretreatment requires advanced treatment techniques such as ion exchange, and the cost is high; though sodium chloride crystals and sodium sulfate crystals are obtained respectively, it is proved through analysis and test that some high-concentration brine waste water has to be released to ensure the purity of the resultant crystalline salts with that method; that is to say, zero release can't be realized with that method; in addition, the salt recovery efficiency attained with that method should be further improved.
The patent document CN105000755A has disclosed a “zero-release” industrial waste water treatment system and a treatment method. The method comprises: pretreating the streams of waste water, and then feeding the pretreated water into a “ultrafiltration+reverse osmosis” unit for desalting treatment by reverse osmosis, so as to ensure that the water quality meets the criteria for reuse; treating the strong brine produced in the reverse osmosis by biochemical treatment to remove concentrated COD and ammonia nitrogen substances in the water; feeding the treated water into a second “ultrafiltration” unit for ultrafiltration; then feeding the treated water into a monovalent and bivalent nanofiltration separation device to separate monovalent sodium chloride and bivalent sodium sulfate; feeding the resultant two streams of water into a second reverse osmosis unit and a third reverse osmosis unit for desalting treatment respectively, to ensure the fresh water meets the criteria for reuse; treating the water outputted from the second reverse osmosis unit and the third reverse osmosis unit by frequent changing-pole electrodialysis; returning the water produced through electrodialysis to the monovalent and bivalent nanofiltration separation device, and treating the concentrated water by evaporating crystallization.
In general, further optimization is required for the existing treatment process of brine waste water, especially brine waste water in the coal chemical engineering industry, in the aspect of comprehensive technical effects, including waste water recovery efficiency, industry salt conversion to resources, salt recovery efficiency, stable operation of membrane unit, reduction of product cost, and realization of zero release, etc.